Vegetable gardeners often talk about their plants bolting. This simply means that the plant sends up a flower stalk and goes to seed. Generally, when plants flower it is considered a good thing, but with vegetables grown for their leaves, like lettuce, spinach, cabbage, and other cole crops, bolting causes the flavor to turn bitter and the leaves to get smaller and tougher, making them inedible. Bolting is very common in cool-season greens, like arugula, lettuce, and spinach. Other common garden plants that bolt include beets and broccoli and herbs such as cilantro, basil, and dill.
Why Does Lettuce Bolt?
Bolting tends to happen when the temperature heats up, but researchers say that it is actually the longer hours of daylight that cause bolting. In one study, researchers covered some plants during the day for different amounts of time and left other plants—the control plants—exposed to a full day of sunlight. Only the control plants bolted. They also tried growing lettuce at high temperatures (90 F.), but with only eight hours of sunlight, and no plants bolted.
In greenhouse studies, lettuce grown in the short days of January didn't bolt until about 135 days after planting, while those sown in July bolted in only 90 days.
Nevertheless, heat may be a factor in bolting if high temperatures occur when the plants are nearing maturity. Dry conditions may also contribute to bolting. Plants that feel threatened by harsh temperatures will often go to seed. Even exposure to cold, while the plants are seedlings, can play a role. If lettuce seedlings are exposed to 40 to 50 F temperatures for several days in a row, they will start forming flower buds, although the flower stalk won't shoot up until the weather warms.
Tips to Prevent Bolting
Unfortunately, there isn't much you can do to stop bolting on lettuce and other plants. Plants that bolt tend to prefer the cooler seasons, and making them happy in the heat of summer will take work.
Some cultivars are quicker to bolt than others. When you're out shopping for seeds, you may see seeds touted as "slow to bolt," but since a variety of conditions can cause bolting, this is no guarantee. That said, if you do have trouble growing lettuce in the summer, it can't hurt to give some of the slow-to-bolt cultivars a try. Many will have names that hint at their bolt resistance, like "Slobolt" and "Summer Bibb." Among the older seed varieties, crisp head lettuces in the romaine or cos group tend to be the slowest to bolt, while looseleaf lettuce tends to be the quickest.
Many gardeners have had success growing lettuce throughout the summer by planting it in a shaded spot in the garden. You can tuck it behind or under taller plants or grow it in pots that can be moved to a shadier site. Regular watering helps to keep the soil cool and the leaves succulent.
A good trick for starting seeds in the summer is to thoroughly soak the area to be planted about two to three days before sowing. After watering, cover the damp soil with a wide board. Repeat this process daily if the weather is particularly hot and dry. Within a couple of days, the soil under the board will be cooler than the surrounding soil. Sow your seeds, then water and cover the soil again with the board. Check daily for signs of germination. At the first sight of green sprouts, remove the board.